The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services. Payload systems for such spacecraft may include high power microwave radio frequency (RF) components such as travelling wave tube amplifiers (TWTA's) and feed networks that are connected by waveguides to radiating elements such as horn antennas and antenna feed elements.
For any mode of transmission of a microwave signal in a waveguide, the electric and magnetic transverse fields may each be resolved into a respective set of tangential and radial components. For a circular waveguide, for example, the tangential and radial components may vary periodically in amplitude along a circular path which is concentric with the wall of the waveguide and may also vary in amplitude along any given radius in a manner related to a Bessel function of order ‘m’. Propagating modes of a transverse electric field are identified by the notation TEmn and propagating modes of a transverse magnetic field are identified by the notation TMmn, where m represents the total number of full period variations of either the tangential or radial component of the respective electric or magnetic field, and n represents one more than the total number of reversals of polarity of either the tangential or the radial component of the respective electric or magnetic field along a radial path.
A mode filter that suppresses one or more undesired propagating modes, while passing one or more other propagating modes is useful for various applications. As an example, application of a mode filter, a circular waveguide having a dominant mode denoted as the TE11 mode, which corresponds to the TE10 mode in rectangular waveguides, may be considered. Waveguides may provide a low-loss transmission path for microwave signals in the dominant TE11 for a circular waveguide or TE10 mode for a rectangular waveguide. It is often desirable to confine the energy propagated in a waveguide to the dominant mode, particularly near an interface between the waveguide and a radiating feed element or horn antenna. Accordingly, there arises a need to suppress TM modes generally, and higher order TE modes.
Higher order modes may result from use of waveguides having a cross-section that is large relative to a wavelength of the propagated signal, irregularities in the path of the waveguide, and/or lack of symmetries in at least some waveguides. Moreover, in satellite communication systems, at least, it is often necessary to operate the same antenna and associated waveguide at two or more disparate frequency bands. Although, in the lowest of the two or more frequency bands, usually only a single mode can propagate in the waveguide, at the higher frequency bands, other higher propagating modes may exist. This can compromise the radiation pattern of the antenna, particularly in terms of cross polarization.
It is a common practice to utilize four-fold symmetry in the feed networks of such antennas to suppress those unwanted modes. However, this results in expensive and big waveguide structures. Therefore, mode filters are desirable to dampen the aforementioned unwanted modes. Mode filters of various types have proven utility for suppressing higher order modes. Such mode filters are disclosed, for example, in U.S. Pat. Nos. 4,222,018, 4,238,747, 4,344,053, and 6,130,586, the disclosures of which are hereby incorporated in their entirety into the present application.
While the mode filters disclosed in the above identified patents may have utility for suppressing higher order modes, the previously disclosed mode filters, in contrast to the present invention, represent a compromise between mechanical and electrical performance. For example, some prior art filters may provide good mode suppression but are relatively bulky, are made of multiple parts, and may be difficult to manufacture and/or integrate. At least some mode filters of the prior art require tuning, and/or provide only narrow band and/or single band mode suppression. At least some known mode filters provide higher insertion loss for main mode and lower attenuation of other propagating modes than the presently disclosed techniques.
More particularly, the previously disclosed techniques have used one or a combination of the following features: dielectric materials and/or materials that are electromagnetically absorptive; resistive and/or lossy material as a coating for internal waveguide surfaces or as an internal load; iris-loaded multimode waveguides; coupling of absorptive waveguides/cavities, loaded with electromagnetically absorptive material, to an overmoded waveguide; provisions for specially designed and arranged leaking/radiating slots on a wall of an overmoded waveguide.
Relative to the above mentioned techniques, mode filters in accordance with the present disclosure provide similar or better mode suppression performance, in embodiments that are generally more compact, lighter weight, simpler to manufacture, and that avoid use of dielectric materials.